LIGHT REACTION'S OF THE MEDUSA GONIONEMUS. 365 



was otherwise darkened on all sides and reflection reduced as 

 much as possible by filtering the water. Now the medusae swam 

 near the bottom, 8 cm. being the greatest height reached. As a 

 control, bottom illumination was then used and immediately one 

 of the animals swam to the surface and one came near it. Three 

 other experiments were made, and the most striking case was 

 that of one medusa swimming through the light band fourteen 

 times in 72 seconds without reaching top or bottom. When 

 the aquarium was so much darkened that I could not see the 

 medusae, sudden illumination showed that they had attached by 

 their tentacles to the sides or bottom. Thus the presence of 

 light seems necessary for the regular up-swimming activity of 

 Gonioncinns and gravity then acts as a stimulus to direct it. 



In regard to the part light plays in the surface reaction, 

 Yerkes 32 says: "... although light seems to be one of the im- 

 portant conditions for this reaction, it may occur in the absence 

 of light." My experiments just cited show that the last part of 

 this statement is not tenable and that the first part is correct. 

 The chief importance of light in bringing Gonioneiinis to the 

 surface, it seems, is in keeping the medusa negative to gravity. 



Morse 33 incorrectly, believing that Yerkes held that light 

 causes the inversion at the surface, denies this, and concludes 

 "the cause for reaction is not evident." Later 34 he explains the 

 inversion the same as Yerkes had previously done 35 by assuming 

 that the apex of the bell is thrust unevenly above the surface. 

 Now as the apex of the bell is heavier we no doubt agree that 

 gravity causes the inversion, though not as a stimulus. 



As the inversion at the surface is preceded by inhibition of 

 contraction the question arises whether this is due to strong 

 light. Yerkes 36 and Morse 37 have observed that medusa do not 

 stop and turn (invert) when made to swim up against a heavier 

 substance than air, such as a board, a glass plate, a layer of olive 

 oil, but they continue to swim against these layers until ex- 



M Jour. of Comp. Neitrol. Psycho!., 1906, Vol. XVI., p. 458. 

 Jour. of Comp. Nenrol. PsychoL, 1906, Vol. XVI., p. 451. 



34 Amer. Nat., 1907, Vol. XLI., p. 686. 



35 Amer. Jour. Physio L, 1903, Vol. IX., p. 281. 



36 Amer. Jour. Physio/., 1903, Vol. IX., p. 281. 



37 Jour. Comp. Neural. Psychol.. 1906, Vol. XVI., pp. 450, 451. 



